Question: What was the first prime-time cartoon?
Answer: Assuming you mean the first prime-time animated network series, Ralph, that would be ABC's legendary Flintstones, which ran for six years beginning in September 1960 (and has been repeated in more places and at more times than I could begin to count). The show pioneered the half-hour animated comedy on network TV, and enjoyed a wave of merchandise sales decades before anyone ever threw on a Bart Simpson "Don't have a cow, man!" T-shirt.
The Flintstones was the fourth animated series created by William Hanna and Joe Barbera (after Ruff and Reddy, Huckleberry Hound and Quick Draw McGraw, all of which were either Saturday-morning or syndicated offerings). And according to Barbera,